The present invention relates to a measuring sensor for determining heat flux through a solid medium, such as, for example, the cooled walls of a heating rod, the measurement being effected by arranging two thermoelements at a defined distance to effect a measurement of a temperature gradient of the mwedium, from which the heat flux is determined.
In many cases, heat flux in the interior or on the surface of bodies must be determined experimentally. Aside from the very integral method of determining a heat balance, there exist basically two different methods: measurement of the temperature gradient; and measurement of the temperature at one point, and determination of the heat flux by solving the so-called inverse heat conduction problem.
If, for example, the heat flux which is transmitted from the wall of a tube to a medium flowing through the interior of the tube is to be measured by measurement of the temperature gradient, two separate thermoelements are usually installed in the wall of the tube at different distances from its interior surface. The heat flux is calculated from the difference between the two temperatures, the distance between the points of mesurement and the heat conductivity of the tube wall. In many cases, however, this method fails because the dimensions are too small and the attainable accuracy is insufficient.
If, for example, a heat flux of 5 W/cm.sup.2 is to be determined by means of two thermoelements disposed at a distance of one millimeter, the temperature to be measured in a stainless steel wall is about 2.5.degree. K. If at a temperature of 500.degree. C. each one of the two temperatures is determined with an accuracy of .+-.1 per thousand, this tolerance alone results in an inaccuracy in the heat flux of .+-.40%. An additional inaccuracy is produced in the determination of the distance between the measuring locations, which without damage can be determined only roughly, and, even when a transverse cut is made in the tube wall, can be effected with an accuracy of .+-.0.05 mm, at most. Added to this is the interfering effect which the thermoelements have on the heat flow. For these reasons, this method can be used with sufficient accuracy only with large heat fluxes and relatively thick-walled tubes.
The method based on measurement of the temperature at one point and determination of the heat flux by solution of the inverse heat conduction problem can be used only for one-dimensional problems.
If, for example, the transient heat flux at the surface of a rod is to be measured, a thermoelement is usually installed in the outer layer of the rod and is used to measure the temperature directly below the surface of the rod as a function of time. The resulting temperature profile in the rod is calculated, taking into consideration the rod structure, including possible contact resistance between various layers
The heat flux is then varied in steps over a period of time to permit determination of that heat flux for which the temperature calculated for the site of the thermoelement coincides with the measured temperature. The solution of this inverse heat conduction problem often produces numerical instabilities which can be avoided only at considerable expense and which adversely influence the accuracy of the measurement.
However, the most significant adverse influence on the accuracy of this method is a lack of knowledge of the thermal behavior of the rod. Particularly the contact resistances pose a great uncertainty.